Hi, I have a Luxman LV 103 amp with A & B switched speaker outlets. When used with headphones the speakers are not muted. Would I be able to bridge each pair of B terminals with say an 8ohm resistor so that I could select the B outputs and listen using headphones without disturbing the Old Dragon and without damage to my trusty old amp?

Unlike vacuum tube amplifiers, the overwhelming majority (probably over 99.999%) of solid state amplifiers do not need to work into a load, you can simply switch to speaker b with the terminals not connected to anything. Vacuum tube amplifiers can sustain damage if not properly loaded because power is reflected back to the output tube plates. You can connect a load if you want to but be careful, you should buy load resistors rated for the wattage of your amplifier if you want to emulate an actual speaker load. For example if you want to load a 100 wpc amp with an 8 ohm load, you should buy resistors rated for 100 watts or more or build a network of resistors that is equivalent. Be warned, when you turn the gain up to listen to your headphones, the resistors will likely get very hot. Be careful they are not near anything that could ignite or be damaged by high temperatures.